This invention relates generally to improvements in methods for forming three-dimensional objects from a fluid medium. More particularly, the invention relates to a new and improved stereolithography method involving the application of enhanced data manipulation and stereolithographic curing techniques to produce three-dimensional objects that are formed more accurately and economically from photocurable polymers.
Stereolithography represents an expeditious way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link to CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes.
Objects built using stereolithography have a tendency to distort from their CAD designed dimensions. This distortion may or may not appear in a specific object, based on how much stress is developed by the specific cure parameters and on the ability of the object to withstand stress. The stress that causes distortion develops when material that is being converted from liquid to solid comes into contact with and bonds to previously cured material. When material is converted from liquid to solid, it shrinks slightly. All stereolithography reins undergo shrinkage when cured. The shrinkage will vary with the type of resin, rate of exposure, and other related factors. This shrinking causes stress and has two primary physical causes: (1) density of the liquid is less than that of the solid plastic; and (2) the chemical reaction that causes the change of state is strongly exothermic, causing the curing material to thermally expand and contract.
Certain sections of an object will be able to resist stresses without any apparent warp, that is where stress is at a tolerable level. On the other hand, other sections may distort considerably as the stress and structural strength balance each other. Since stress is caused by contact between curing material and cured material, it can be propagated along the entire length of contact between the curing line and cured material. Most contact of curing to cured material occurs from one layer to the next as opposed to along a single layer. This implies most distortions will be vertical in nature as opposed to horizontal. Further, large features in the XY plane tend to shrink more than smaller features in the XY plane. Since shrinkage is a fixed volumetric percentage, when a part transitions from a large feature to a small feature along the Z direction, the difference in shrinkage values produces a pronounced discontinuity on the external surface that is a geometric imperfection more commonly known as the differential shrinkage effect. Differential shrinkage is greatest in parts transitioning from a large feature to a small or thin feature, for example, from a large base to a thin wall. Therefore, there is a need for a technique to reduce vertical distortions.
It is an aspect of the present invention that a method of producing stereolithographic parts is disclosed that uses a curing technique which reduces or controls differential shrinkage to an acceptable level in the building of parts.
It is another aspect of the present invention that shrinkage during the building of stereolithographic parts is controlled in the Z direction.
It is a feature of the present invention that a build style is used that combines a cyclic build style with a small spot size curing weave on layers of build material not being cyclically hatched or cured.
It is another feature of the present invention that the small spot size curing reduces the amount of material cured at one time, thereby reducing the rate of shrinkage in the part.
It is yet another feature of the present invention that the curing technique provides a grid formed from the small spot size curing that acts as a simulated fiber in the fiber-reinforced plastic to prevent or reduce further material shrinkage.
It is yet another feature of the present invention that the solidifying layers of build material in any intermediate partially cured layers are not adhered to the underlying layer after a curing done in a first scanning direction and are adhered to the underlying layer after a curing done in a second scanning direction.
It is still another feature of the present invention that use of the small spot size laser curing permits the cure to be deeper and slightly slower without appreciably affecting the speed of part formation.
It is an advantage of the present invention that the stereolithographic method of curing a photopolymer material reduces distortion in transition areas going from large features to small features.
It is another advantage of the present invention that the speed of the build of a stereolithographic part is not sacrificed to an unacceptable level to achieve reduced differential shrinkage in the part.
These and other aspects, features and advantages are achieved in the stereolithographic method of the present invention by curing multiple layers of build material in a cyclical fashion by solidifying a first fully cured layer of build material, partially solidifying at least one partially cured intermediate layer of build material, and then solidifying a second fully cured layer of build material and the liquid areas of unsolidified build material in the at least one intermediate partially cured layer.